This invention relates to a method of producing boron carbide and more particularly to a method of producing boron carbide of submicron size.
Boron carbide (B.sub.4 C) is a ceramic material which is used in application requiring great hardness. For example, boron carbide is used in armor plating applications and for producing sand blasting nozzles, bearings and dies. For some applications, it is important and desirable to use high purity, monodispersed boron carbide powder having a size of less than one micron because such powders, for example when subjected to a hot-pressing process for forming ceramic products, yield a higher quality ceramic part.
There are a number of methods in the art for producing a boron carbide powder. For example, U.S. Pat. No. 2,834,651 discloses a method of producing fine boron carbide of fine particle size by heating a mixture of boron oxide, carbon and magnesium. The boron carbide produced by the method described in U.S. Pat. No. 2,834,651 is unsatisfactory for high purity applications because the boron carbide is contaminated with the magnesium starting material and even after repeated digestions with hot mineral acids, the magnesium is difficult to remove.
Very fine powders of boron carbide have been produced by vapor phase reactions of boron compounds with carbon or hydrocarbons, using laser or plasma energy sources. These reactions tend to form highly reactive amorphous powders. Due to their extreme reactivity, handling in inert atmospheres may be required to avoid excessive oxygen and nitrogen contamination. These very fine powders have extremely low bulk densities which make loading hot press dies and processing greenware very difficult.
It is desired to prepare a boron carbide powder as a single phase equiaxed crystalline product with a narrow particle size distribution so as to have optimum reactivity. Said product could be hot pressed into a pore free, uniform fine grained B.sub.4 C ceramic product without excess carbon or low melting metallic carbide impurities which are deleterious to physical properties of the final ceramic product.
Another method known in the art for producing a boron carbide powder is described in U.S. Pat. No. 3,379,647. That method involves a carbothermic reduction of boron oxide. According to said method, a reactive mixture comprising a carbon source, such as finely divided carbon, and a boron oxide source, such as a boron oxide, is prepared and then fired at a relatively high temperature, whereby the boron oxide which is present initially or which is formed thereupon is reduced, the corresponding boron carbide being concurrently produced. This reaction ordinarily proceeds according to the general equation: EQU 2 B.sub.2 O.sub.3 +7C.fwdarw.B.sub.4 C+6 CO
Generally, the temperature of firing the reactive mixture above is in the range of 1700.degree.-2100.degree. C. The reaction is generally carried out in a protective, non-interfering atmosphere such as an inert gas or a vacuum. A major shortcoming of the method of U.S. Pat. No. 3,379,647 is that substantially all of the product is not below one micron and a uniform size distribution is not obtained. In the aforementioned process, the particle size of boron carbide can range anywhere from 0.5 to 150 microns with no control of particle size distribution.
It is desired to provide a novel method of producing a submicron size boron carbide powder by the carbothermic reduction of an oxide of boron. It is further desired to obtain a boron carbide product wherei substantially all of the particles of boron carbide are less than 1 micron and wherein at least about 95 percent by count of the particles are less than 1 micron.